Or when Maidenform just isn’t enough.
LiveScience reports that Bras Don’t Support Bouncing Breasts.

The science of brassiere design in this study refers to the biomechanics of breasts bouncing during exercise. For women with larger endowment, e.g., a pair of D-cup breasts weighing 15-23 pounds, the unrestrained movement has potential to damage the delicate supporting ligaments and inflict pain. Women may abandon active sports because of this.
Sports bras of the compression type only limit up-and-down movement of the breasts. The study revealed that during vigorous exercise, women’s breasts not only move up-and-down, but also side-to-side and in-and-out. The sum of the motion is something akin to a figure eight.

While brassieres have evolved throughout history from body-binding corsets to cleavage-enhancing “miracle” bras, only recently have researchers injected a dose of science into the design of undergarments that go beyond conferring a more “perky” look, the researcher says.
“It is only recently that bra design has turned to science,” said study author Joanna Scurr, a biomechanics professor at the University of Portsmouth in England. “There was no research. It’s like designing a car or kitchen equipment without first thinking ‘what is the purpose of this?’”

A figure eight. Huh.